Molecular Geometry Notes
February 3, 2014
• You will be able to describe the molecular
geometry of a molecule.
• Correct HW 6B
• A note on nomenclature
• Notes
• Cool video of the day: Check out THIS ionic crystal
lattice!!
Binary Nomenclature
• First element by name (with prefix, ONLY
if there is more than one atom)
• Second element as an “-ide”, with prefix
• Does not work for molecules with more
than 2 atoms
•
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CO
NO
SF2
SO2
NO2
NCl3
CCl4
H2O
N2F6
N2O5
Carbon monoxide
Nitrogen monoxide
Sulfur difluoride
Sulfur dioxide
Nitrogen dioxide
Nitrogen tricloride
Carbon tetrachloride
Dihydrogen monoxide
Dinitrogen hexafluoride
Dinitrogen pentoxide
Molecular Geometry
• Applies to any Lewis diagram. (Involved in
covalent bonding)
VSEPR Theory
• Valence Shell Electron Pair Repulsion
• Each bonding group or lone pair of electrons
spreads out as far as possible from every
other bonding group or lone pair of electrons.
• Think 3D!
Electron Pair vs. Molecular Geometry
• Electron pair geometry describes both bonds
AND lone pairs located around the central
atom.
• Molecular geometry describes only the bonds
located around the central atom.
• Molecular geometry is a subset of electron
pair geometry.
Practice
 How many bonding groups are
around the central atom?

4
 How many lone pairs around
the central atom?

0
Practice
 How many bonding groups are
around the central atom?

3
 How many lone pairs are
around the central atom?

1
Practice
 How many bonding groups are
around the central atom?

3
 How many lone pairs are
around the central atom?

0
Practice
 How many bonding groups are
around the central atom?

2
 How many lone pairs are
around the central atom?

0
Electron Pair Geometry: Overview (Not in notes)
• The shape of the molecule including
both bonds and lone pairs
• Linear
– 2 e- groups from central atom
• Trigonal Planar
– 3 e- groups from center
• Tetrahedral
– 4 e- groups from center
Any of these
lobes (electron
groups) can be
either lone pairs
or covalent
bonds
REMEMBER
• ELECTRON PAIR GEOMETRY and MOLECULAR
GEOMETRY are not the same thing.
– EPG describes all electron groups (including lone
pairs)
– MG describes only bonding groups
2 Electron Groups
• Electron pair geometry: Linear
– 2 bonding groups
– 0 lone pairs
– Molecular geometry: Linear
– Example: CO2
• 2 bonding groups
• 0 lone pairs
Double and triple
bonds count as
just ONE bonding
group!!!
3 Electron Groups
• Electron pair geometry: Trigonal Planar
– 3 bonding groups
– 0 lone pairs
– Molecular geometry: Trigonal planar
– Example: NO3This Lewis structure also
shows the molecular shape
(trigonal planar).
Double and triple
bonds count as
just ONE bonding
group
3 Electron Groups
• Electron pair geometry: Trigonal Planar
– 2 bonding groups
– 1 lone pair
– Molecular geometry: Bent
– Example: NO2Lone pair is one of the
three electron groups
around the central
atom
Double and triple
bonds count as
just ONE bonding
group
4 Electron Groups
• Electron pair geometry: Tetrahedral
– 4 bonding
– 0 lone pairs
– Molecular geometry: Tetrahedral
– Example: CH4
This Lewis structure does NOT
show the tetrahedral molecular
shape. It DOES show the four
electron groups.
4 Electron Groups
• Electron pair geometry: Tetrahedral
– 3 bonding
– 1 lone pair
– Molecular geometry: Trigonal Pyramidal
– Example: NH3
This Lewis structure does
NOT show the molecular
shape. It DOES show the
four electron groups.
Lone pair is one of the
four electron groups
around the central
atom
4 Electron Groups
• Electron pair geometry: Tetrahedral
– 2 bonding groups
– 2 lone pairs
– Molecular geometry: Bent/Angular
– Example: H2O
This Lewis structure shows
both the molecular shape
and the four electron
groups
Lone pairs are two of
the four electron
groups around the
central atom
Molecular Geometry: Overview
• The shape of the molecule including just bonds between
atoms.
• 2 e- groups from central atom:
– If 2 bonds/0 lone pairs: Linear
• 3 e- groups from central atom:
– 3 bonds/0 lone pairs: Trigonal planar
– 2 bonds/1 lone pair: Bent
• 4 e- groups from central atom:
– 4 bonds/0 lone pairs: Tetrahedral
– 3 bonds/1 lone pair: Trigonal pyramidal
– 2 bonds/2 lone pairs: Bent
Sketches of Molecular Geometry
• Sketches (on homework, quizzes, and tests)
should show the shape of the molecule in
three dimensions:
Exceptions to the Octet Rule
(SF , BrF , XeF , IF )
(Beyond the scope of this class)
6
5
4
7